The present invention relates to a method of cleaning valves or lines through which hydrolysable polymers are transported at the operating temperature.
The term “hydrolysable polymers” here is taken to mean thermoplastic polyesters, polyamides or polycarbonates, such as polyethylene terephthalate or naphthalate, polypropylene terephthalate or naphthalate, polybutylene terephthalate or naphthalate, polyamide 6 or 6.6, poly(bisphenol A carbonate) or copolymers thereof.
The preparation and processing of polymer melts or high-viscosity polymer solutions frequently requires the polymer stream to be split into sub-streams, for example if a plurality of processing positions are connected simultaneously. The flow splitting is usually carried out by means of a plurality of valves, where each individual valve may be closed or open intermittently. In order to maintain the flowability of the polymer solutions and particularly of the polymer melts, high operating temperatures of up to about 300° C. are necessary, resulting, with the valve closed, in decomposition of the polymer residues remaining in the valve to give carbon-like products. Even closed polymer valves may develop leaks at the seats due to design and production flaws and damage during start-up or in operation. This may result in total blockage of the following line. After a re-start, the polymer decomposition products are entrained by the polymer stream, contaminating the fresh polymer, which then inevitably has to be discarded or at best can be converted into low-quality products.
It is known that polymer filters can be cleaned by treatment with steam (DE 196 49 013 A) or a mixture of steam and an oxidising gas (EP 0 791 386 A) in the filter housing or in a closed tank after removal. However, the cleaning does not extend to the polymer valves and lines adjacent to the filter, which instead have to be uncoupled from the cleaning of the filter to be cleaned.
The object of the present invention is to indicate a method which enables the cleaning of valves or lines for hydrolysable polymers, where the cleaning should be carried out as far as possible in the installed state, without extensive assembly work.